Abstract: ESD testing
sometimes seems to be unrepeatable, depending of the phase of the moon
and other seemingly unrelated conditions. There are many reasons for
this, but for air discharge events, the nature of the spark in air
leads to significant variability in test results. Typical high frequency measured current
waveforms are presented for 8 kV and 15 kV air discharges and the
implications discussed.

Discussion: Figure 1 shows an example of the discharge current of a 15 kV air discharge from an EM Test ESD 30N ESD Simulator
as measured with a Fischer Custom Communications F-65 current probe.
The discharge was made by extending the test tip of the simulator
through the current probe and discharging in air to the Ground
Reference Plane of an IEC 61000-4-2 test setup. The ESD 30N is ideal
for this as the test tip extends far enough through the current probe
so the discharge reliably goes to the Ground Reference Plane instead of
the current probe body.

The discharge has a double peak reaching about 32 Amperes and an
initial rise to the first peak of about 5 nanoseconds. The fall time
after the second peak is approximately 100 nanoseconds, about as expected.

In Figure 2, the current waveform of a second air discharge at 15
kV is shown. Note there is only one peak of only about 23 Amperes and
the initial rise is much slower at about 20 nanoseconds.

Compare the waveforms in Figures 1 and 2 to Figure 3 where the
risetime is about 10 nanoseconds reaching a single peak of about 29
Amperes. And Figure 4 shows yet another case of much a longer risetime
of about 30 nanoseconds to a peak of about 31 Amperes. These four
waveforms were selected out of a set of 15 waveforms and are typical of
the rest of the set with some variations.

Figure 5 shows an example of an 8 kV air discharge. The presence of the
"hash" on the waveform is due to EMI from a very fast risetime
affecting the scope. The risetime is likely faster than 1 nanosecond
to do this. The peak current is about 20 Amperes, not that much less
than the 23 Ampere peak resulting from a 15 kV discharge in Figure 2.

But in Figure 7, we see a slower,
smoother waveform reaching only about 16 Amperes peak. Figure 8 has
about a 5 nanosecond initial rise with two peaks, the highest at about
15 Amperes. The area under the curve of Figure 8 is less than Figure 5,
6, or 7 so the I2t energy is less also.

From the data above, it appears there is quite a bit of variation
between air discharge events at 8 kV and above. A possibly
oversimplified way of looking at this is that long sparks involve many
collisions between electrons crossing the gap and air molecules. The
electrons get scattered and are pulled in by the field with a
relatively slow risetime on average with
a lot of variation from spark to spark. Small, low voltage sparks
result in fewer air-electron collisions and the risetime is therefore
faster.

Air discharges are also subject to variation with air pressure,
temperature, and humidity as well. The result is that ESD testing using
discharges in air can result in a range of test results, especially at
high
voltages

Summary: Air
discharge testing, especially at 8 kV and above, can produce a lot of
variation in current waveforms and test results from discharge to
discharge.

I would like to thank RMV Technology Group at NASA Ames Research Center for use of their facilities to generate the data for this Technical Tidbit.

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